The present invention relates to a slippage control method and device for a four wheel drive power transmission system for a vehicle, and more particularly relates to such a slippage control method and device for such a four wheel drive power transmission system for a vehicle such as an automobile adapted for four wheel drive operation, particularly adapted to control the differential action of a differential device which is provided for distributing power between the front wheels of the vehicle and the rear wheels of the vehicle, in which the construction and operation thereof are improved so as to improve the quality of slippage control and thereby improve vehicle drivability.
The present invention has been described in Japanese Patent Application Ser. Nos. 61-009100 and 61-121438 (1986), filed by an applicant the same as the entity assigned or owed duty of assignment of the present patent application; and the present patent application hereby incorporates into itself by reference the text of said Japanese Patent Application and the claims and the drawings thereof; a copy is appended to the present application.
Nowadays a greatly increasing number of automotive vehicles are being constructed with four wheel drive transmission systems, because such four wheel drive operation, in which all four wheels of the vehicle are powered from its engine via its transmission, is very suitable for driving on poor or slippery road surfaces such as in mud or over bad ground, or upon roads covered with mud, snow, ice, or rain. In other words, four wheel drive operation provides a much higher degree of stability and drivability for the vehicle in conditions where the coefficient of friction between the wheels and the surface upon which the vehicle is riding is relatively low. Also, four wheel drive operation is beneficial for aiding with hill climbing characteristics and high speed stability characteristics. Therefore, the so called full time four wheel drive type of transmission, which remains always engaged to four wheel drive without any episodes of two wheel driving, is becoming more and more popular.
In such a four wheel drive transmission system for an automotive vehicle, it is usual to provide a center differential device for distributing rotational power between the front wheels of the vehicle and the rear wheels of the vehicle, as well as the per se conventional rear differential device that provides differential action between the two rear vehicle wheels and the also per se conventional front differential device that provides differential action between the two front vehicle wheels. Such a central or front-rear differential device is provided in order to provide a differential action between said front vehicle wheels (considered as a pair) and said rear vehicle wheels (also considered as a pair) when the vehicle is turning around a curve, in order to eliminate the possibility of the occurrence of the so called tight corner braking phenomenon created by the difference in the turning radiuses of the front wheels of the vehicle and the rear wheels thereof. And such provision of such a central or front-rear differential device is effective for achieving this result. However, a problem that arises with such provision of such a central or front-rear differential device is that, if at any time even one only of the four vehicle wheels should break away from the road surface and should start to spin, then no drive power or at least severely reduced drive power is provided to the other three vehicle wheels. This type of problem is particularly likely to arise in the event that the road conditions are poor due to rain, snow, dust, dirt, or the like which deteriorate the coefficient of the vehicle wheels on the road surface, and thereby vehicle drivability can be severely reduced.
In order to counteract this effect, it has been practiced to provide a device to such a front-rear differential device which prevents said front-rear differential device from performing differential action, in a selective fashion. When such a center differential action inhibition means, which typically may be a hydraulic clutch or a hydraulic brake, is actuated, it causes the differential action provided by said front-rear differential device between the front vehicle wheels and the rear vehicle wheels to be prevented, and instead said front vehicle wheels, considered as a pair, are driven from the vehicle engine, and also said rear vehicle wheels, considered as a pair, are independently driven from said vehicle engine. Thereby, the problem outlined above, of loss of power to the other three vehicle wheels when one of the vehicle wheels starts to spin, is obviated. Such types of structure are disclosed, for example, in Japanese Utility Model Application Laying Open Publication Ser. No. 47-203 (1972), Japanese Patent Application Laying Open Publication Ser. No. 50-147027 (1975), and Japanese Patent Application Laying Open Publication Ser. No. 60-176827 (1985), none of which is it intended hereby to admit as prior art to the present patent application except to the extent in any case required by applicable law.
In the event that the front-rear differential device is of an unequal distribution type which distributes drive torque substantially unequally between the front vehicle wheels and the rear vehicle wheels, then, during periods in which said front-rear differential device is not being prohibited from providing its differential action by the above mentioned differential action inhibition means, the amounts of torque distributed between the front vehicle wheels and the rear vehicle wheels are different. In the case that the amount of torque distributed to the rear vehicle wheels is larger than the amount of torque distributed to the front vehicle wheels, the performance of the vehicle for starting off from rest is improved; while, in the converse case that the amount of torque distributed to the front vehicle wheels is larger than the amount of torque distributed to the rear vehicle wheels the performance of the vehicle for straight ahead driving operation, and the stability of such straight ahead driving operation, are improved.
The following type of problem, however, can tend to arise in a so called full time four wheel drive vehicle fitted with such a front-rear differential device equipped with such a differential action inhibition means. When the transmission of such a vehicle is shifted between its speed stages--either in the case that said transmission is an automatic transmission or in the case that said transmission is a manual transmission but particularly in the former case that said transmission is an automatic one--this is typically done in accordance with variation of various vehicle operational parameters such as vehicle road speed and vehicle engine load (typically throttle opening), and in particular down shifts from a higher speed stage to a lower speed stage are typically carried out during episodes of vehicle acceleration and during vehicle progress along upwardly sloping road surfaces. In these types of operational conditions, the torque transmitted by the power output shaft of the engine (typically its crank shaft) increases, and accordingly the torque transmitted by the power output shaft of the transmission is increased by an even greater degree according to the shifting of the speed stage provided by said transmission from said higher speed stage to said lower speed stage, as a result of which there is a rather high likelihood of the driving vehicle wheels slipping on the surface of the road, and there is an according danger that vehicle driving performance and vehicle drivability may be deteriorated, and further that vehicle controllability may suffer. Further, when the vehicle is equipped with a typical type of automatic transmission which has various transmission operation ranges such as "D" range, "2" range, "L" range which are manually changed between by the vehicle driver but within which selection of a speed stage is automatically performed, in the case of travel along a downwardly sloping road surface, when the vehicle driver changes the transmission operational range from the "D" range to the "2" range in order to obtain an improved engine braking effect, i.e. changes the transmission operational range from a higher driving range to a lower driving range, and when even further said vehicle driver further changes the transmission operation range down from the "2" range to the "L" range in order to obtain yet further improved engine braking effect, the transmission performs automatic downward shifts of speed stage in accordance with these manual transmission range changes. In order that an automatic downward speed stage shift prompted by such a manual transmission range change should be completed as early as possible, the speed stage shift is carried out relatively quickly, and hence the torque on the output shaft of the transmission changes relatively abruptly in accordance with this speed stage shift. As a result of this, it is particularly easily in these operational circumstances for the driving vehicle wheels to slip on the surface of the road, and there is an according danger that full braking for the vehicle, particularly engine braking, may not be available, and that the vehicle driving performance and vehicle drivability and vehicle controllability may be deteriorated.
Further, in the case that the front-rear differential device is of an unequal distribution type which distributes drive torque substantially unequally between the front vehicle wheels and the rear vehicle wheels, then--either in the case that said transmission is an automatic transmission or in the case that said transmission is a manual transmission--when a down shift of the vehicle transmission is carried out as a result of vehicle deceleration when the vehicle is turning around a curve, the following further problems can tend to occur. In the case that the amount of torque distributed to the rear vehicle wheels is larger than the amount of torque distributed to the front vehicle wheels, the cornering force provided by said rear vehicle wheels will be reduced as a result of this transmission down shift, and as a result the vehicle will have a tendency to oversteer; while, in the converse case that the amount of torque distributed to the front vehicle wheels is larger than the amount of torque distributed to the rear vehicle wheels, the cornering force provided by said front vehicle wheels will be reduced as a result of this transmission down shift, and as a result the vehicle will have a tendency to understeer. In either case, the vehicle stability, controllability, and maneuverability will be deteriorated. Further, the higher the vehicle speed becomes, the more conspicuous do these effects become.